Angular ball bearing and rolling bearing

ABSTRACT

A rolling bearing is proposed in which even though a sealed type, no lowering of strength of the bearing rings or lowering of the front width will not occur for the convenience of mounting of a seal, and design of dimensions equivalent to a non-sealed type bearing and practicability can be achieved, and has fretting resistance. Rolling elements  3  are disposed between an inner ring  1  and an outer ring  2  with seals  9  and  10  provided. The seal  9  arranged on a counterbore  8  of the outer ring  2  is fitted in a pressed state on a peripheral surface portion of a cylindrical surface  11  forming the counterbore  8.  By pressing, the seal  9  is mounted without forming a slot. The other seal  10  is fitted in a slot  12.  The inner ring and the outer ring, or rolling elements of a bearing comprising the outer ring, inner ring, rolling elements and a retainer are subjected to carbonitriding and a grease using a urea compound as a thickening agent is sealed in the bearing.

TECHNICAL FIELD TO WHICH THE INVENTION BELONGS

This invention relates to angular ball bearings and rolling bearingswith a seal used in general purpose industrial machinery at a supportportion for a spindle of a machine tool or a ball screw support portion.

PRIOR ART

As shown in FIG. 8, an angular ball bearing has a large outer diameterD3 of the inner ring 51 on its loaded (front) side, and a small innerdiameter D4 of the outer ring 52 on its loaded (back) side so that ahigh axial load can be borne. The inner diameter D2 of the inner ring 51on its non-loaded side, and the inner diameter D5 of the outer ring 52on its non-loaded side are provided with tapered portions called“counterbores” formed by cutting off one of the shoulders of eachraceway groove for assemblability. As a result, the outer diameters D2and D3 of the inner ring, and the inner diameters D4 and D5 of the outerring are markedly different from each other.

For such an angular ball bearing, too, according to the intended use orform of use, as with a deep groove ball bearing, it is sometimes desiredto be of a sealed type. Sealing means is provided to prevent lubricantretained in the bearing from leaking outside, and to prevent entry ofdust and water into the bearing from outside. In order to make anangular ball bearing into a sealed type, as with an ordinary sealed ballbearing, as shown in FIG. 9, seals 55, 56 are fixed to the outer ring 52in slots 57, 58. The seals 55, 56 may be of a contact type in which lipsat their tips contact the inner ring 51 and noncontact type which do notcontact.

Among machine tools, as shown in FIG. 12, there is one in which a ballscrew 72 is rotated by a motor 71 to reciprocate a table 73 by therotation of the ball screw 72. Such a machine tool is used to machine aworkpiece 74 on the table 73 with a blade 75 while reciprocating thetable 73.

In this arrangement, the ball screw 72 is usually supported by a bearing76. As the bearing 76, an angular ball bearing is usually used.Lubrication is by grease or oil.

But in a bearing for supporting a ball screw for a machine tool,vibrating loads are often applied due to increase in vibratingcomponents of machining loads produced during machining. With increasein such vibrating loads, fretting wear damage may develop on the rollingsurfaces of such a support bearing. Such fretting wear damage developsdue to minute sliding, minute rolling, minute vibration, etc and has alarge influence on deterioration of the bearing performance such as poorrotation accuracy (tone).

In contrast, there is disclosed in JP patent publication 7-92104 amethod in which the rolling surfaces are subjected to carbonitriding toincrease the hardness of the rolling surfaces and improve the wearresistance.

PROBLEMS THE INVENTION INTENDS TO SOLVE

In the conventional angular ball bearing shown in FIG. 9, since theportion 52 a of the outer ring 52 where the counterbore is formed isthin in the diametrical thickness, if the slot 57 for fixing the seal 55is formed, the wall thickness of the outer ring 52 becomes too thin atits portion where the slot 57 is formed. This lowers the strength of theouter ring 52. Also, if the slot 57 is formed, since the innerperipheral portion 60 from the slot 57 to the outer ring width surfacehas a reduced diameter to introduce the seal 55, the front surface widthW of the outer ring 52 becomes narrow. The width surface 61 of the outerring 52 is a portion which serves for axial positioning of the bearingand is also a portion for bearing the pre-load. If a plurality of suchbearings 70 are arranged as shown in FIG. 10, large loads are applied tothe width surface 61 of the outer ring 52. Such an arrangement isusually employed for bearings for the support portion of e.g. a ballscrew. For these reasons, insufficient front surface width W of theouter ring 58 will cause such trouble as the deformation of the outerring 52 and poor positioning of the bearing due to decreased strength,and insufficient pre-load. While the front surface width W is influencedby dimensions of various parts of the bearing, if the slot 57 is formedat the portion of the counterbore 59, it is difficult to ensure thefront surface width W.

For example, an angular ball bearing is designed so that (balldiameter)/(outer ring outer diameter−ball pitch circle diameter)=R willbe in the range of 0.4-0.7. If the rate R is less than 0.4, the loadbearing capability would be low compared with the size of the bearing.This is not economical. Conversely, if R is 0.7 or over, the balls 53occupy the bearing cavity too much to ensure a sufficient wall thicknessfor the inner and outer rings 51, 52. For a bearing in which the rate Ris near the lower limit 0.4 but 0.44 or over, if the slot 57 is formedas shown in FIG. 9, it is difficult to ensure a sufficient front surfacewidth W for the outer ring 58.

To prevent this, one may think of reducing the ball pitch circlediameter PCD and the ball diameter d to reduce the outer ring innerdiameter D5 on the front side (FIG. 8), thereby increasing the wallthickness of the outer ring front side. But with this arrangement,compared with a non-sealed type bearing having no seals, the rigidity ofthe support portion, the load bearing capacity of the bearing, and therolling fatigue life would lower. In particular, in an angular ballbearing used to support a threaded shaft of a ball screw in a machinetool, such lowering of the load bearing capacity and rolling fatiguelife is problematic. That is to say, reduced rigidity of the machinetool feed unit leads to lowering of the machining accuracy. This is abig disadvantage. As for the load bearing capacity and the rollingfatigue life, too, like the rigidity, their lowering is a bigdisadvantage.

On the other hand, due to the influence of internal design, it issometimes difficult to ensure space for the seal 55 and its mountingportion with the same main dimensions as with a non-sealed type bearing.In such a case, one may think of extending the width B of the bearing.But since the main dimensions are changed, compatibility with anon-sealed type bearing is lost. This is not economical.

A first object of this invention is to provide an angular ball bearingwhich even though it is a sealed type, does not suffer from reduction inthe strength of the bearing rings and lowering of the front surfacewidth for convenience of mounting of the seals, and allows to achievedesign of dimensions equivalent to those of a non-sealed type bearingand practicality.

In recent years, for machine tools such as machines for making molds,machining into more complicated shapes is required. For working intosuch complicated shapes, microscopic feed is needed, and the morecomplicated the shape is, the greater the frequency of microscopic feed.Thus, on rolling surfaces of a rolling bearing at a support portion fora ball screw used in a machine tool, pivoting motion is frequently used,so that microscopic rolling frequently occurs. Thus, only applyingcarbonitriding is insufficient, and fretting wear damage often develops.

Therefore, a second object of this invention is to provide a rollingbearing having resistance to fretting even if used at a support portionfor a ball screw of a machine tool for machining into complicated shapesin which the frequency of microscopic feed has increased.

MEANS TO SOLVE THE PROBLEMS

In the angular ball bearing of this invention, rolling elements aredisposed between an inner ring and an outer ring, a seal is provided onat least on one side thereof, the seal on at least one side is fitted ina pressed state on a peripheral surface of a counterbore formed bycutting off one of shoulders of a raceway groove of the inner ring orthe outer ring. That is, a seal is fitted in a pressed state on a flatperipheral surface portion of the counterbore.

With this arrangement, the seal provided on the peripheral surface ofthe bearing ring formed with a counterbore is fitted in a pressed stateon the peripheral surface without forming a slot. Thus the seal can bemounted without locally thinning the wall thickness of the bearing ringat its portion where the counterbore is formed or reducing the frontsurface width. Mounting of the seal by pressing on the flat peripheralsurface without forming a slot is made possible by suitably adjustingthe sectional shape of the seal e.g. by expanding the axial width of theportion of the seal where it is pressed, e.g. by providing a shortcylindrical portion on one side of the seal remote from the seal lipportion. Since the seal is fitted not in a groove, it is possible todesign a sealed type angular ball bearing having main dimensions, suchas ball pitch circle diameter, ball diameter, etc. which are equivalentto those of a non-sealed type bearing. As a result, an angular ballbearing is provided which is high in reliability with respect to leak oflubricant to outside, invasion of powder, dust, water, etc. into thebearing from outside. Also, since it has parts, dimensions of which areequivalent to those of a non-sealed type bearing, it can be designedequivalently in rigidity of the support portion, load bearing capacityand fatigue life. Thus, it has compatibility with a non-sealed typebearing.

A cylindrical seal mounting surface may be formed on the peripheralportion of the counterbore of the inner ring or the outer ring to fitthe seal in a pressed state on the seal mounting surface.

If the seal mounting surface is a cylindrical surface, control ofinterference for pressing is easy, so that stable pressing is possible.If the peripheral surface portion is a tapered surface, even if thecylindrical seal mounting surface is formed, unlike a slot, influence onthe wall thickness of the bearing ring or front surface width is small.Thus it is possible to provide a bearing ring having a wall thicknessand a front surface width that are practically equivalent to thearrangement in which the seal mounting surface is not cylindrical.

According to this invention, the bearing may have a seal on the frontside, fitted in a pressed state on the peripheral portion of thecounterbore on the front side of the outer ring, and a seal on thebackside fitted in a slot on the inner peripheral surface of the outerring on the backside. If a seal is mounted in a slot, the axial width ofthe mounting portion may be narrow. Like on the backside, at a portionwhere the axial width from the raceway groove of the outer ring to thewidth surface is narrow but the diametrical wall thickness is thick,there will be no shortage of the mounting surface. Thus a seal can bemounted without causing a problem of lowered strength due to theformation of a slot. Thus, by pressing a seal onto the flat peripheralsurface portion at a portion where the outer ring is thin, and bymounting it in a slot at a portion where it is axially narrow and thickin the diametric direction, mounting of seals can be done easily andrigidly without any influence on the bearing rings.

In this invention, the dimensions of the parts may be in the followingrelations. That is, (bearing width)/(inner diameter of innerring)=0.2-1.0, and (bearing width)/(ball diameter)=1.5-2.2.

Among the major dimensions of an angular ball bearing, such as the innerring outer diameter, outer ring outer diameter, bearing width or height,and chamfer dimension, some are standardized by the InternationalStandardization Organization (ISO), but others are not. In either case,(bearing width)/(inner diameter of inner ring)=S is 0.2-1.0. If S isless than 0.2, it is impossible to employ a sufficient ball sizerelative to the bearing size, so that no sufficient load-bearingcapacity can be obtained. Conversely, if S exceeds 1.0, the space whichthe bearing occupies increases, which increases the entire device. Thisis not economical. (bearing width)/(ball diameter)=T is 1.5 to 2.2 underthe above standardization. If T is less than 1.5, the rate at which theballs occupy in the bearing cavity increases. This will make itdifficult to ensure a sufficient wall thickness of the bearing rings.Conversely, if T exceeds 2.2, the load-bearing capacity is low comparedto the bearing size. This is uneconomical. In a bearing of which therates S and T are in the above preferable ranges, it is possible toemploy a structure in which a seal is mounted by pressing on the flatperipheral surface portion which is the counterbore in this invention.Thus the abovesaid functions and effects due to this mounting structureare revealed.(ball diameter)/(outer diameter of outer ring−ball pitch circlediameter) may be 0.44 or over.

The angular ball bearing used for a ball screw in a machine tool is, asdescribed in the section about problems the invention intends to solve,designed with the value (ball diameter)/(outer diameter of outerring−ball pitch circle diameter)=R in the range of 0.4 to 0.7. Even ifthe rate R of the balls is close to the lower limit 0.4, for a bearingin which this value is 0.44 or over, if a slot is formed as in the priorart, it is difficult to ensure a sufficient front surface width on theouter ring.

Thus, in conventional bearings, in order to make it possible to mount aseal, the ball occupying rate R had to be reduced to the limit, therebyrestricting the load bearing capacity. Since this invention employs astructure in which the seal is pressed on the flat peripheral surfaceportion, while it has seals, it can be designed with a large balloccupying rate R to 0.44 or over, and thus a large load bearing capacityis assured.

The angular ball bearing of this invention may be used for supportingthe threaded shaft of a ball screw.

Since high axial loads are applied to an angular ball bearing used forsupporting the threaded shaft of a ball screw, a bearing having a largecontact angle e.g. 30° or over, i.e. a bearing that is high in the axialload bearing capacity is required. If the contact angle increases, thecounterbore deepens correspondingly, so that the front surface width ofthe bearing ring decreases. Even in a bearing having such a largecontact angle, by employing the seal mounting structure according tothis invention by pressing on the peripheral surface portion, there areno lowering of strength and lowering of the front surface width, it canbe mounted in this way.

Also, the second object of this invention is solved by using a rollingbearing which comprises an outer ring, an inner ring, rolling elementsand retainer, and in which the inner ring and the outer ring, or therolling elements are subjected to carbonitriding, and a grease using aurea compound as a thickening agent is sealed in the bearing.

The inner ring and outer ring, or the rolling elements are subjected tocarbonitriding, and a grease using a urea compound as a thickening agentis sealed in the bearing. As a result, a thin oxide film of a ureacompound is formed, and an oil film having a sufficient thickness isformed on the oxide film. Since this thin oxide film of a urea compoundis high in adhesion to the carbonitrided layer, even if micro rollingoccurs frequently, grease is retained on the rolling surfaces, so thatit is possible to effectively prevent lowering of durability bysuppressing fretting damage on the raceway surfaces. Also, even if oilfilm of the grease between the rolling elements and the raceway surfacesbreaks, so that the rolling elements and raceway surfaces contactdirectly, due to the function of the carbonitrided layer provided on therolling elements or raceway surfaces, development and progression offretting damage is suppressed for a while. Also, since oil film of thegrease is high in adhesion to the carbonitrided layer, even if breakageoccurs, it is repaired quickly. Thus, even if the oil film breaks andthe rolling elements and raceway surfaces directly contact, it ispossible to repair the oil film before fretting damage develops. Thus,resistance to fretting damage markedly improves.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially sectional view of the angular ball bearing of oneembodiment of this invention,

FIG. 2 is a partially sectional view of the same embodiment incomparison with a conventional bearing of the same size,

FIGS. 3(A) and 3(B) are sectional views showing arrangements of theangular ball bearings of the same embodiment,

FIGS. 4(A) and 4(B) are sectional views showing other arrangements ofthe angular ball bearing of the same embodiment,

FIG. 5 is a sectional view showing a feed mechanism of a machine toolusing the angular ball bearing,

FIG. 6 is a partially sectional view showing the angular ball bearing ofanother embodiment of this invention,

FIG. 7 is a partially sectional view showing the angular ball bearing ofstill another embodiment,

FIG. 8 is a sectional view of a conventional nonsealed type angular ballbearing,

FIG. 9 is a sectional view of a conventional sealed type angular ballbearing,

FIG. 10 is a sectional view of an example of parallel arrangement ofconventional angular ball bearings,

FIG. 11 is a sectional view showing an example of the angular ballbearing according to this invention,

FIG. 12 is a schematic view showing an example of a machining deviceusing a bearing for supporting a ball screw,

FIG. 13 is a schematic view showing a bearing used in Example 1 andComparative Examples 1-3,

FIG. 14 is an assembly view for conducting Fafnir fretting corrosiontest,

FIG. 15 is a graph showing the results of a microscopic pivoting weartest,

FIGS. 16 and 17 are graphs showing the results of a microscopic slidewear test, and

FIGS. 18 and 19 are graphs showing the results of an accumulatedbreakage probability experiment.

EMBODIMENTS OF THE INVENTION

The first embodiment of this invention will be described with referenceto the drawings. In this angular ball bearing, a plurality of rollingelements 3 are disposed between raceway grooves 5 and 6 of an inner ring1 and an outer ring 2 as bearing rings. These rolling elements 3 areretained in pockets 4 a of a retainer 4. Compared with deep groove ballbearings, the inner ring 1 has a larger outer diameter on the loadedside (front side F), while the outer ring 2 has a smaller inner diameteron the loaded side (backside B) to create a contact angle θ between theraceway grooves 5 and 6. For the sake of assembling the bearing, acounterbore 7 where one of the shoulders of the raceway groove 5 is cutoff is formed on the non-loaded side of the inner ring 1. A counterbore8 where one of the shoulders of the raceway groove 6 is cut off isformed on the non-loaded side of the outer ring 2, too.

In the bearing cavity between the inner and outer rings 1 and 2, seals 9and 10 are arranged. The seal 9 on the front side F is pressed into theinner peripheral surface 11 of the outer ring 2 which forms thecounterbore 8. The seal 10 on the backside B is fitted in a slot 12formed in the inner-diameter surface of the outer ring 2. These seals 9and 10 may be contact seals or non-contact seals, but are contact sealsin this embodiment.

The inner peripheral surface 11 forming the counterbore 8 of the outerring 2 is a conical surface, and a portion outside of the bearing is acylindrical seal mounting surface 11 a. The seal 9 on the front side hasa short cylindrical fitting portion 9 a at its proximal end, i.e.opposite a seal lip portion. The fitting portion 9 a is pressed into theseal mounting surface 11 a. Since the seal 9 on the front side ispressed with interference, it will not fall after press-fitting. Thefitting force between the seal 9 and the outer ring 2 is freelyselectable by adjusting the interference. The short cylindrical fittingportion 9 a is cylindrical with its outer-diameter surface parallel tothe axial direction. The seal 9 has a core metal 14 to which is fixed anelastic member 15. The seal lip portion 9 b formed by an elastic member15 is in contact with the outer peripheral surface of the inner ring 1.The core metal 14 has an L-shaped section with one edge forming thefitting portion 9 a. Part of the elastic member 15 covers the outerperiphery of the short cylindrical portion at the proximal end of thecore metal 14 in contact with the seal mounting surface 11 a. But thecore metal 14 may be directly fitted on the seal mounting surface 11 a.Also, the outer-diameter surface of the fitting portion 9 a may not be acylindrical surface but be tapered. Even if it is tapered, the seal 9can be fixed. If it is tapered, the seal mounting surface 11 a of theouter ring 2, too, is a tapered surface. For example, it is a surfacecontiguous with the other portion of the inner peripheral surface 11.

The seal mounting surface 11 a may be finished by lath turning orgrinding. At the edge of the seal mounting surface 11 a opposite thepress-fitting side, a recess 13 is formed to allow a lathing tool and agrinder to be removed. While the recess 13 is a kind of groove, since itis not used to fix the seal 9 on the front side, it is shallow in groovedepth and will not result in insufficient wall thickness of the outerring 2. The shape of the seal lip portion 9 b can be arbitrarilydetermined, and either of the contact type and noncontact type may beused.

The seal 10 on the backside has an elastic member integrally fixed to acore metal and fitted in the slot 12 of the outer ring 2. The portion 16of the outer ring 2 from the slot 12 to its inner edge has an increasedinner diameter to introduce the seal 10 on the backside. The shape ofthe seal lip portion 10 b of the seal 10 on the backside may also beeither the contact type or noncontact type.

The peripheral surfaces of the inner ring 1 and outer ring 2 opposingthe seal lip portions 9 b and 10 b of the seals 9 and 10 are flat inthis embodiment. But they may be formed with oil grooves in which theseal lip portions 9 b and 10 b of the seals 9 and 10 may contact or beloosely fitted. The lubrication of the bearing may be by grease or oil.

With this arrangement, since the seals 9 and 10 are provided on thefront side and backside, respectively, it can be a sealed type. Thus itcan be an angular ball bearing which is high in the reliability andprevents leak of the lubricant and entering of the dust and water intothe bearing from outside. Since the seal 9 on the front side provided atthe peripheral surface portion 11 formed with the counterbore 8 of theouter ring 2 is fitted in the inner peripheral portion 11 by pressing.Thus it can be fitted without the need of forming a slot. The shape ofthe seal 9 on the front side can be of any shape so long as the fixedportion can be mounted on the flat seal mounting surface 11 a. The shapeof the seal lip portion 9 b can be arbitrarily determined, and may beeither contact type or noncontact type. Since the shape of the seal lipportion 9 b of the seal 9 on the front side is no different from that ofa conventional groove-fitted type seal, the function of preventing leakof lubricant and the function of preventing dust, debris or water fromentering into the bearing from outside will not lower. Although the seal10 on the backside is fitted in the slot 12, since the outer ring 2 isthick in the wall thickness on the backside, there will be no problem oflowering the strength due to the provision of slot 12. Since thebackside is narrow in the axial width from the raceway groove 6 of theouter ring 2 to the end face, by providing the slot 12, it can be easilymounted in a narrow space.

Since the seal 9 on the front side can be fixed at the portion of theouter ring 2 where the counterbore 8 is formed, without forming agroove, it is possible to prevent local reduction in the wall thicknessat the abovesaid portion of the outer ring 2. It is also possible toensure the front surface width W. Also, both the seals 9 and 10 on thefront side and backside can be fixed in the bearing internal space inthe case of a non-sealed type bearing. Thus, there will be no change inwidth even if it is of the sealed type.

Thus, with the angular ball bearing of this embodiment, it is possibleto design a sealed type bearing having the same main dimensions, ballpitch circle diameter PCD, and ball diameter d as a non-sealed typebearing. Thus, the reliability improves in preventing leak of thelubricant to outside and preventing dust, debris and water from enteringthe bearing. Also, it is possible to achieve the rigidity of supportportions, load bearing capacity and rolling fatigue life, that areequivalent to those of a non-sealed type bearing. Thus, the angular ballbearing of this embodiment is compatible with a non-sealed type bearing.

FIG. 2A is a view showing an angular ball bearing in which the bearingof this embodiment is applied to an angular ball bearing for supportinga ball screw, and FIG. 2B shows a conventional angular ball bearing inwhich seals are fixed in slots, by arranging them side by side with thesame dimensions for comparison sake. As for the bearing size, the innerring inner diameter is 75 mm, the bearing width is 20 mm, and thecontact angle is 60° for both.R=((ball diameter)/(outer diameter of outer ring−ball pitch circlediameter)=0.549

If a sealed type angular ball bearing (FIG. 2(B)) is designed with aconventional arrangement in which a slot 57 is formed, as described inthe section describing problems the invention intends to solve, thefront surface width W of the outer ring 52 on the front side, and thewall thickness of the outer ring 52 at the portion of the counterborewould be too small at the portion of the slot 51, trouble such asdeformation of the outer ring 52 and poor bearing positioning, andinsufficient pre-load will be caused.

On the other hand, in the embodiment of this invention (FIG. 2(A)), theseals 9 and 10 are both fixed to the outer ring 2 with the seal 10 onthe backside fixed in the slot 12 similar to a conventional one and theseal 9 on the front side fixed by pressing to form a sealed typebearing.

In this case, there is no reduction in the front surface width W of theouter ring on the front side and the wall thickness of the outer ring atthe counterbore portion 8 as in the conventional arrangement. Thus itwill be a practicable design.

In this embodiment (FIG. 1, FIG. 2(A)), axial positioning of the seal 9on the front side is controlled by the difference between the widthsurface 17 of the outer ring on the front side and the width surface ofthe seal 9. But positioning may be done by pushing the seal 9 into theboundary between the recess 3 and the counterbore portion 11.

Angular ball bearings for supporting a ball screw often form a supportportion by combining a plurality of rows, for example, by using in twoor three rows as shown in FIGS. 3(A), 3(B), or in four rows as shown inFIGS. 4(A) or 4(B). In the example of FIG. 3(A), two rows of angularball bearings 20 are arranged with their fronts facing each other tobear axial loads in one row. In the example of FIG. 3(B), two (two rightrows in the figure) of the three rows have their fronts facing eachother, and the remaining row arranged facing the same direction as theadjacent one to bear axial loads in two rows. In the example of FIG.4(A), the central two rows have their front facing each other with therows on both sides thereof in the same direction as the adjacent one(so-called DFTT combination) to bear axial loads in three rows. In theexample of FIG. 4(B), two at one end (right in the figure) of the fourrows have their fronts facing each other and the remaining two rows inthe same direction as the adjacent one (DTFT combination) to bear axialloads in three rows.

When angular ball bearings 20 are arranged in a plurality of rows inthis manner, large loads are applied to the width surface W of the outerring 2 on the front side (ditto for the width surface of the inner ring1 on the backside). But with the angular ball bearings 20 of thisembodiment, since the front width W of the outer ring 2 is sufficientlyensured, they can be used even for applications in which they arecombined in a plurality of rows as described above. Besides, since theyhave main dimensions, rigidity of support portions, load bearingcapacity and rolling fatigue life which are equivalent to those of anon-sealed type bearing, replacement of a non-sealed type bearing withthe sealed type bearing of this embodiment will result in no functionaltrouble.

FIG. 5 shows a feed mechanism of a machine tool using this angular ballbearing. A table 31 is reciprocably mounted on a base 32 through a guide(not shown), and is reciprocated by driving a motor 33 through a ballscrew 34. The ball screw 34 has a nut 35 mounted to the table 31 and athreaded shaft 36 rotatably supported on the base 32 at support portions37 and 38 at both ends thereof. The threaded shaft 36 is coupled to themotor shaft 33 a of the driving motor 33 through a coupling 39. Thesupport portions 37, 38 support the threaded shaft 36 through rollingbearings 20, 41 provided in housings 37 a and 38 a. For the rollingbearing 40 of the support portion 37 on the side of the driving motor33, the angular ball bearings 30 of the above embodiment are used. Forthe support portion 37, angular ball bearings 20 are arranged in aplurality of rows as shown in FIGS. 3(A), (B), FIGS. 4(A), (B).

Next, the dimensional relation of parts of the angular ball bearing 20of the first embodiment shown in FIG. 1 will be described. Thisdimensional relation is an example when e.g. it is applied to a bearingfor supporting a ball screw used in a feed line of a machine tool.

In this angular ball bearing,S=(bearing width)/(inner diameter of inner ring)=0.2 to 1.0T=(bearing width)/(ball diameter)=1.5 to 2.2Also,R=(ball diameter)/(outer diameter of outer ring−ball pitch circlediameter)≧0.44Also,R≦0.7

The contact angle θ is 30° or over.

The main dimensions of rolling bearings refer to dimensions that showthe contour of bearings. For international compatibility and economicalproduction, they are standardized by International StandardsOrganization (ISO). In Japan, they are stipulated under JIS B 1512. Themain dimensions are the inner diameter, outer diameter, width or heightof a bearing and chamfering dimensions. These dimensions are importantwhen the bearing is mounted on a shaft and a housing. In principle,dimensions concerning the internal structure are not stipulated. Whilemany dimensions of rolling bearings are stipulated, they are forpreparation of the future standardization, and those actually used noware not all of these dimension groups.

As described above, while there are standardized ones andnon-standardized ones among the main dimensions for rolling bearings, ineither case, (bearing width)/(inner ring inner diameter)=S is 0.2 to1.0. If S is less than 0.2, it is impossible to employ a sufficient ballsize relative to the bearing size. Thus a sufficient load bearingcapacity is not obtainable. Conversely, if S exceeds 1.0, the bearingoccupying space increases, so that the entire device becomes bulky. Thisis uneconomical.

Also, (bearing width)/(ball diameter)=T is preferably 1.5 to 2.2 for theabove standardization and the like. If T is less than 1.5, the rate atwhich the balls occupy in the bearing cavity increases, so that itbecomes difficult to ensure a sufficient wall thickness of the racewayring. Conversely, if T exceeds 2.2, the load bearing capacity will below compared with the bearing size. This is uneconomical. With a bearingof which the rates S and T are in preferable ranges, a structure can beemployed in which the seal 9 is mounted by pressing into the flatperipheral surface portion which forms the counterbore 8 in thisembodiment. In such a case, the above functions and effects arerevealed.

For an angular ball bearing used e.g. for a ball screw of of a machinetool, the value of (ball diameter)/(outer ring outer diameter−ball pitchcircle diameter)=R is, as described in the section of problems theinvention intends to solve, usually set in a range of 0.4 to 0.7. If therate R is 0.4 or less, the load bearing capacity would be low for thebearing size. This is uneconomical. Conversely, if R is 0.7 or over, therate at which the balls occupy in the bearing cavity would increase.This makes it difficult to ensure sufficient wall thicknesses of theinner and outer rings. In a bearing in which even though the balloccupying rate R is close to the lower limit 0.4, if it is 0.44 or over,a slot 57 is formed as in the conventional example of FIG. 9, it becomesdifficult to ensure a sufficient front surface width W of the outer ring58.

Thus, in a conventional bearing, in order to make it possible to mountseals, it was necessary to decrease the ball occupying rate R as much aspossible, thereby restricting the load bearing capacity. In thisinvention, since a seal is pressed into the flat peripheral surfaceportion, it is possible to increase the ball occupying rate R to 0.44 orover. Thus, design which permits a larger load bearing capacity ispossible.

As for the contact angle θ, to a bearing for supporting a ball screw ofa machine tool, since high axial loads are applied, the bearing shouldhave a contact angle θ of 30° or over. Namely, the angular ball bearing30 has a high axial load bearing capacity.

In the above embodiment, the seal 10 on the backside is fitted on theouter ring 2 using the slot 12. But as shown in FIG. 6, like the seal 9on the front side, the seal 10A on the backside, too, may be pressedinto the inner peripheral surface of the outer ring 2. In this case, theseal 10A on the backside is, like the seal 9 on the front side, formedinto a shape having a tubular fitting portion 10Aa at the proximal end.In the embodiment of FIG. 6, the seal 10A on the backside has the samesectional shape except that it is different in diameter from the seal 9on the front side.

In the above embodiments, the seals 9, 10, 10A are mounted on the outerring 2. But this invention is also applicable to the arrangement inwhich seals 9B and 10B are fitted on the inner ring 1 as shown in FIG.7. In this case, the seal 9B is pressed on the portion of the inner ring1 where a counterbore 7 is formed.

In the above embodiments, the seals 9, 10, 10A, 9B, 10B have an elasticmember such as rubber fixed to a core metal. But these seals may bemetallic shield plates or of an oil seal type. In any of the aboveembodiments, seals 9, 10, 10A are provided on both sides. But accordingto the intended use, this invention is applicable to an arrangement inwhich a seal is provided only on one side if the seal is fitted on thecounterbore side.

Next, the rolling bearing according to this invention is, as shown inFIG. 11, a bearing comprising an outer ring 83, an inner ring 81,rolling elements 87 and a retainer 88, and in which cabonitrided layers85 are provided by subjecting the inner ring 81 and the outer ring 83 tocarbonitriding, and grease 86 using a urea compound as a thickeningagent is sealed in the bearing. Also, while not shown in FIG. 11, arolling bearing in which carbonitrided layers 85 are formed on therolling elements 87 instead of on the inner ring 81 and the outer ring83 is included in the rolling bearing of this invention.

As material for the outer ring 83, inner ring 81 and rolling elements87, SUJ2, SUJ3 or steel containing C: 0.1-1.0 wt %, Mn 0.1-1.0 wt %, Cr0.1-20 wt %, the balance being Fe and unavoidable impurities, etc. canbe cited.

The carbonitriding is one of the means for hardening a metallic surface.The carbonitriding is applied because with ordinary carburization it ispossible to obtain a tough material, but it is unstable to heat. Incontrast, by nitriding, the material surface is hardened and theresidual austenite becomes stable to heat, so that the material becomesresistant to impact. Further, a suitable amount of carbide deposits, sothat it is possible to increase the fatigue strength without loweringthe resistance to cracking. Such a carbonitrided layer 85 may be formedon the inner ring 81 and outer ring 83, or on the rolling elements 87.

As the carbonitriding method, after carbonitriding in a high-temperaturegas in which ammonium gas is added to a carburizing atmosphere, thematerial may be hardened and tempered.

The amount of the residual austenite is preferably 20-40%. If less than20%, improvement in the rolling fatigue life may not be sufficient. Onthe other hand, if over 40%, the hardness of the carbonitrided layer maydecrease, so that the wear resistance properties lower.

The grease 86 contains a urea compound as a thickening agent, and a baseoil added thereto. By using a grease of which the thickening agent is aurea compound, a thin oxide film of urea compound is formed on theraceways of the inner and outer rings. On the film, an oil film-having asufficient thickness is formed.

In particular, by applying carbonitriding to the inner and outerraceways, even if oil film of grease between the rolling elements andthe raceways disappears and the rolling elements and the racewaysdirectly contact, due to the function of the carbonitrided layer formedon the raceways or rolling elements, progression of fretting wear damageis suppressed. Also, oil film of grease is high in adhesion to thecarbonitrided layers, even if breakage of oil film occurs, it is quicklyrepaired. Thus, due to the synergistic effects of the grease and thecarbonitrided layer, it is possible to suppress fretting wear andeffectively prevent lowering of the durability.

The urea compound may be aliphatic, cycloaliphatic or aromatic, and canbe used by mixing them at an arbitrary rate. Among them, a diurea orpolyurea expressed by the following formula 1 is preferable. Among them,diurea is preferable.

(Formula 1)

(wherein R2 is an aromatic hydrocarbon, aliphatic hydrocarbon orcycloaliphatic hydrocarbon group having a carbon number of 6-15, R1 andR3 are aromatic hydrocarbon groups having a carbon number of 6-12,cyclohexyl groups, cyclohexyl derivatives having a carbon number of 7-12or alkyl groups having a carbon number of 6-20.

As the base oil, one or more lubricating oils selected from mineral oil,synthetic hydrocarbon oil and ether oil, or a mixed oil mixed at anarbitrary rate may be used. Among them, a mineral oil is preferable.Since a mineral oil has a good compatibility with the urea thickeningagent, good lubricity and suitable consistency, there will be no leak ofgrease, and repairability of the grease film will not be impaired.

The mixing rate of the thickening agent in the urea grease is preferably1-40 wt %, more preferably 5-20 wt %. If less than 1 wt %, gel hardeningof the thickening agent would be insufficient, and the consistencyincrease, so that grease tends to leak. On the other hand, if over 40 wt%, the consistency would lower, worsening the flowability.

To the urea grease, within such a range that the function of the ureagrease will not be impaired, known rust preventives, antioxidants,extreme pressure additives, wear suppressants, oiliness improvers,corrosion inhibitors, pour point depressants, viscosity index improvers,structure stabilizers, thickeners, antistatic agents, emulsifiers andcolorants may be added.

The grease 86 is filled into the bearing, specifically between the innerring 81 and the outer ring 83 as shown in FIG. 11 so as to cover therolling elements 87 and the retainer 88. If a sealed type bearing isused as the rolling bearing, it is sealed by seals 89 and 90 at thefront side and the backside of the bearing. If the sealed type rollingbearing is used, it is possible to prevent the grease 86 from leakingout. Instead of the seals, shield plates may be used.

The two seals 89 and 90 have different shapes. With this arrangement, itis easy to confirm the bearing mounting direction. Thus it is possibleto prevent assembling error.

As the kind of rolling bearing according to this invention, it is notspecifically limited, but is preferably an angular ball bearing.

The rolling bearing according to this invention can be used at a supportportion of a ball screw, particularly a support portion of a ball screwused in a machine tool. Among support portions of ball screws of themachine tool, if it is used at a support portion of a ball screw of amachining center for machining of complicated shapes in which thefrequency of microfeed is high, it can effectively reveal the frettingresistance. This is more preferable.

EXAMPLES

Hereinbelow, more detailed description is made with reference toExamples. First in Examples 1 and 2, as judgment about resistance tofretting wear damage, tests were conducted for micro sliding wear andmicro pivoting wear.

[Carbonitriding Treatment]

Using inner ring and outer ring plates made of SUJ2, their rollingsurfaces were subjected to carbonitriding. For carbonitriding, they wereheld for 40 minutes at 880° C. in a continuous furnace in which 10%ammonium gas in volumetric ratio was added to NX gas. Next, they weresubjected to tempering at 180° C. for two hours to obtain carbonitridedplates.

The surface hardness (HRC) of the plates obtained was 63.2. The HRCbefore treatment was 61.9.

Example 1 Micro Pivoting Wear Test

The outer ring 90 a and inner ring 90 b that have been subjected to theabovesaid carbonitriding, and rolling elements 90C (made of SUJ2) wereassembled to manufacture a bearing 91 (inner ring inner diameter×outerring outer diameter×width=20 mm×40 mm×14 mm). At this time, 1 g of ureagrease described in Table 1 was sealed (grease is not shown in FIG. 13).Using this bearing 91, Fafnir fretting corrosion test was conductedunder ASTM D 4170. Specifically, as shown in FIG. 14, the outer ring 90a and the inner ring 90 b were fixed to two bearing retaining portions96, and a shaft 93 was passed to a bolt 92 in the order shown. And byadjusting the tightening of the bolt 92, a load was applied by a spring94 (load=2.45 kN). With a chuck portion set in a tester, a pivotingportion 95 and a motor were coupled together by a crank rod, and themotor was rotated in the atmosphere at room temperature. With thepivoting angle set to 12 deg (critical pivoting angle: 30 deg) and thepivoting cycle set to 30 Hz, the test was conducted for eight hours. Interms of weight reduction of the inner and outer rings after the test,the wear property of the plates was evaluated. The results are shown inFIG. 15.

Comparative Example 1

Except that an inner ring and an outer ring made of SUJ2 and notsubjected to carbonitriding, and a lithium-family grease shown in Table1 was used, a microscopic pivoting wear test was conducted in the samemanner as in Example 1. The results are shown in FIG. 15.

Comparative Example 2

Except that as a grease, a lithium grease shown in Table 1 was used, amicroscopic pivoting wear test was conducted in the same manner as inExample 1. The results are shown in FIG. 15.

Comparative Example 3

Except that as a grease, a urea grease shown in Table 1 was used, amicroscopic pivoting wear test was conducted in the same manner as inExample 1. The results are shown in FIG. 15.

(Results)

From Example 1 and Comparative Examples 1-3, it became apparent that bysubjecting the rolling surfaces to carbonitriding and using a ureacompound as a grease, microscopic pivoting wear, namely, wear produceddue to microscopic rolling markedly decreases. Thus, it became apparentthat it had resistance to fretting wear resulting from it.

Example 2 Microscopic Slide Wear Test

With rolling elements (made of SUJ2) placed on a plate subjected to thecarbonitriding, and using the urea grease described in Table 1 as agrease, micro slide wear tests were conducted by the following method.As for the test conditions at this time, the load applied to the rollingelements was 98 N, amplitude of the rolling elements on the plate was0.47 mm, the frequency was 30 Hz, the number of loadings was 8.6×10⁵cycles, and the test time was eight hours.

And at 5 or more points and in a direction perpendicular to the weardirection, measurement was made on a Talysurf surface profiler and thedeepest value was used as the plate wear depth. Also, the wear amount(γ) of the rolling elements was calculated by measuring the weardiameter with a microscope and using the following formula. The resultsof plate wear depth are shown in FIG. 16, and the results of wear amountof the rolling elements are shown in FIG. 17.ν=(πh ²×(3r−h))/3h=r−(4r ² −c ²⁾ ^(1/2)/2wherein γ: radius of the ball, h: wear depth of the ball, c: diameter ofwear marks

Comparative Example 4

Except that an inner ring and an outer ring made of SUJ2 and notsubjected to carbonitriding, and a lithium grease shown in Table 1 wereused, a microscopic slide wear test was conducted in the same manner asin Example 2. The results of plate wear depth are shown in FIG. 16, andthe results of wear amount of the rolling elements are shown in FIG. 17.

Comparative Example 5

Except that as a grease, a lithium grease shown in Table 1 was used, amicroscopic slide wear test was conducted in the same manner as inExample 2. The results of plate wear depth are shown in FIG. 16, and theresults of wear amount of the rolling elements are shown in FIG. 17.

Comparative Example 6

Except that as a grease, a urea grease shown in Table 1 was used, amicroscopic slide wear test was conducted in the same manner as inComparative Example 4. The results of plate wear depth are shown in FIG.16, and the results of wear amount of the rolling elements are shown inFIG. 17. TABLE 1 grease viscosity of base oil thickening (mm²/s) brandmaker agent consistency base oil 40° C. 100° C. urea Pyronic Nissekiurea 289 mineral 108.5 11.9 grease Universal Mitsubishi oil N6C lithiumAlvania SHOWA lithium 277 mineral 131.5 9.9 grease No. 2 Shell oil(Results)

From Example 2 and Comparative Examples 4-5, it became apparent that bysubjecting the rolling surfaces to carbonitriding and using a ureacompound as a grease, microscopic slide wear, namely, wear produced dueto microscopic sliding markedly decreases. From Example 2 andComparative Example 6, it became apparent that Example 2 had asufficient micro slide wear resistance.

Example 3

An inner ring 81 and an outer ring 83 made of SUJ2 were subjected to thecarbonitriding and assembled with rolling elements made of SUJ2 tomanufacture the angular ball bearing shown in FIG. 11. And as a grease,urea grease shown in Table 1 was sealed. A load of 6.9 kN (radial load)was applied to the bearing and it was rotated at 2000 rpm. Theaccumulated breakage probability 10% life at this time was measured. Theresults are shown in FIG. 18.

The “Accumulated breakage probability 10% life” refers to thesubstantially total number of revolutions or operating hours duringwhich 90% (reliability 90%) of identical bearings in a group can berotated without producing flaking due to rolling fatigue when they areindividually rotated under the same conditions.

Comparative Example 7

Except that an inner ring 81 and an outer ring 83 made of SUJ2 weresubjected to the through hardening, the accumulated breakage probability10% life was measured in the same manner as in Example 3. The resultsare shown in FIG. 18.

Through hardening is a treatment in which after a bearing steel has beenheated to and held at 800-850° C., it is cooled rapidly. This forms amartensitic composition in the steel, so that the material is hardened.

Example 4

Except that foreign matter was mixed in the urea grease described inTable 1 used as a grease, the accumulated breakage probability 10% lifewas measured in the same manner as in Example 3. The results are shownin FIG. 19.

Comparative Example 8

Except that an inner ring 81 and an outer ring 83 made of SUJ2 weresubjected to the through hardening, the accumulated breakage probability10% life was measured in the same manner as in Example 4. The resultsare shown in FIG. 19.

(Results)

From Examples 3 and 4 and Comparative Examples 7 and 8, it becameapparent that the angular ball bearing obtained by subjecting therolling surfaces to carbonitriding and using a urea compound as a greasehad a sufficient durability.

EFFECT OF THE INVENTION

With the angular ball bearing of this invention, since rolling elementsare disposed between the inner ring and the outer ring, a seal isprovided at least on one side, and the seal is fitted on the peripheralsurface portion of the counterbore in a pressed state, even though it isof a sealed type, lowering of the strength of the bearing rings orreduction of the front width for the convenience of mounting of the sealwill not occur. Thus design is possible for dimensions andpracticability equivalent to a non-sealed type bearing.

Also, with the bearing according to this invention, since the inner ringand the outer ring or the rolling elements are subjected tocarbonitriding and the grease using a urea compound as a thickeningagent is sealed in the bearing, a thin oxide film of a urea compound isformed and a sufficiently thick oil film is formed on the oxide film.Because the thin oxide film of urea compound is high in adhesion to thecarbonitrided layer, even if microscopic rolling occurs frequently,grease is held on the rolling surfaces, so that it is possible tosuppress fretting damage on the rolling surfaces, thus preventinglowering of the durability effectively.

Further, even if oil film of the grease between the rolling elements andthe rolling surfaces should break, so that the rolling elements androlling surfaces contact directly, due to the function of thecarbonitrided layers provided on the rolling elements or the rollingsurfaces, production and progression of fretting damage are suppressedfor a while. Also, since oil film of grease is high in adhesion to thecarbonitrided layers; even if breakage of film occurs, it is quicklyrepaired. Thus, even if oil film breaks and the rolling elements androlling surfaces directly contact, oil film can be repaired beforefretting damage occurs. Thus, resistance to fretting damage improvesgreatly.

Also, since this bearing for supporting a ball screw has a seal, thebearing interior is sealed. Thus, scattering of grease, which wasobserved in a conventional open type bearing, is suppressed.

Further, it is possible to prevent entering of foreign matter such ascoolants into the grease from the atmosphere.

Still further, since two kinds of seals are of different shapes, it iseasy to confirm mounting directions when assembling the bearing. Thus itis possible to prevent mis-assembling.

1-6. (canceled)
 7. A rolling bearing comprising an outer ring, an innerring, rolling elements and a retainer, wherein said inner ring and saidouter ring, or said rolling elements are subjected to carbonitriding anda grease using a urea compound as a thickening agent is sealed in saidbearing.
 8. A rolling bearing as claimed in claim 7 wherein said bearingis an angular ball bearing.
 9. A rolling bearing as claimed in claim 7wherein said bearing is a sealed type bearing.
 10. A rolling bearing asclaimed in claim 7 which is used for a support portion for a ball screw.11. A rolling bearing as claimed in claim 10 wherein said supportportion for a ball screw is used for a machine tool.
 12. A rollingbearing as claimed in claim 8 wherein said bearing is a sealed typebearing.
 13. A rolling bearing as claimed in claim 8 which is used for asupport portion for a ball screw.
 14. A rolling bearing as claimed inclaim 9 which is used for a support portion for a ball screw.